Abstract

Organic photovoltaic devices continue to attract attention as they offer the prospect oflow-cost energy production on flexible substrates. However, there remains a need tounderstand factors which affect device performance at a more fundamental level. Oneparticular area of focus has been related to the device open-circuit voltage (Voc) whose originsare still the subject of debate. This thesis therefore sets out to try and understand and correlatethe electronic properties of materials with critical device parameters, such as the Voc.The experimental determination of the work function, ionisation potential and electronaffinities are important for predicting and understanding the energetics at the interface formedwhen two solids are combined. The offsets at the interface between different oxide /phthalocyanine / fullerene heterojunctions were characterised. It was found that the upperlimit to Voc is governed by the offsets between the HOMO state in the donor and the LUMOstate of the acceptor (the effective bandgap). Furthermore, by understanding the electronicstructure and characterising device performance in the limit of low temperature, it was foundthat the experimental Voc is limited by the reverse saturation current and the exciton bindingenergy - a bottleneck for organic semiconductors.An understanding of the benefits of incorporating molybdenum oxide into a devicearchitecture was achieved by conducting a thorough investigation on its electronic structure.The conduction mechanism is believed to be dictated by a distribution of near Fermi leveldefect states caused by non-stoichiometry. Finally, the effects of tuning the molecularorientation on the device performance were investigated with a flat lying perylene derivative.Optimised device architectures based on the photoactive ClAlPc / C60 heterojunctiondisplayed an efficiency of 3.0 %.